1. Field of the Invention
The present invention generally relates to a solid state image pickup device and a camera, and more particularly to a solid state image pickup device the dynamic range of which is expanded.
2. Description of Related Art
FIG. 7 is a circuit diagram of a solid state image pickup device disclosed in Non-Patent Document 1 (Shigetoshi Sugawa et al. (five persons), “A 100 dB Dynamic Range CMOS Image Sensor Using a Lateral Overflow Integration Capacitor”, ISSCC 2005/SESSION 19/IMAGERS/19.4, DIGEST OF TECHNICAL PAPERS, 2005 IEEE International Solid-State Circuit Conference, Feb. 8, 2005, pp. 352-353, 603.) FIG. 8 is a timing chart showing the operation of the solid state image pickup device. Hereinafter, an n-channel MOS field effect transistor is simply referred to as a transistor. FIGS. 9A, 9B, 9C, 9D, 9E and 9F are conceptual diagrams for illustrating the potential in a series of operations of the solid state image pickup device and the flows of carriers.
First, as shown in FIG. 9A, at timing t2, a transistor M3 is turned on, and noises N2 after a reset are accumulated in a floating diffusion FD and an additional capacity CS. Then, the noises N2 are read.
Next, as shown in FIG. 9B, at timing t3, light is radiated onto a photodiode PD, and carriers are accumulated in the photodiode PD. After the accumulated carriers of the photodiode PD have filled the photodiode PD, the carriers overflowed from the photodiode PD flow into the floating diffusion FD and the additional capacity CS.
Next, as shown in FIG. 9C, after the carrier accumulation ends, at timing t4, the transistor M3 is turned off, and the carriers are distributed to the floating diffusion FD and the additional capacity CS at a predetermined rate. A pixel signal S2 and noises N2′ are accumulated in the additional capacity CS. A pixel signal S1 is accumulated in the photodiode PD.
Next, as shown in FIG. 9D, the floating diffusion FD is reset at timing t5. Only noises N1 remain in the floating diffusion FD. After that, the noises N1 are read from the floating diffusion FD.
Next, in FIG. 9E, a transistor M1 is turned on, and the carriers of the pixel signal S1 in the photodiode PD are transferred to the floating diffusion FD. In the floating diffusion FD, the noises N1 and the pixel signal S1 are accumulated.
Next, in FIG. 9F, the transistor M3 is turned on. The carriers of the pixel signals S1 and S2, and the noises N1 and N2′ are accumulated in the floating diffusion FD and the additional capacity CS, and the carriers are read.
Moreover, Japanese Patent Application Laid-Open No. 2001-186414 (corresponding to U.S. Pat. No. 6,307,195) and Japanese Patent Application Laid-Open No. 2004-335802 disclose solid state image pickup devices for expanding dynamic ranges.
The circuit shown in FIG. 7 is a circuit for one pixel, and it is difficult to use the transistors M2, M4 and M5 commonly to a plurality of pixels in order to decrease the number of the transistors. In the circuit of FIG. 7, the carriers overflowed from the photodiode PD flow into the additional capacity CS through the floating diffusion FD. Consequently, if it is tried to share the transistors M2, M4 and M5, the floating diffusion FD of a plurality of pixels is mutually connected, and the pixel signal of the plurality of pixels is mixed with each other. Hence, it becomes impossible to read the signal of each pixel. Consequently, it is impossible to share the transistors M2, M4 and M5 to decrease the number of the transistors.
Moreover, because the carriers overflowed from the photodiode PD flow into the additional capacity CS through the floating diffusion FD, noises are easily generated due to the influence of the defects (damages) of the floating diffusion FD. The floating diffusion FD has many defects as compared with the photodiode PD. Consequently, in the floating diffusion FD, the accumulated carriers disappear owing to the defects, and great fixed pattern noises occur.
It is an object of the present invention to provide a solid state image pickup device and a camera that can decrease the number of circuit elements and generate pixel signals with fewer noises.